Thrill seekers may want to hitch a ride on the giant comet
Hale-Bopp, but
they would fail vehicle emission tests miserably. When it was the same
distance from the Sun as Earth,
Hale Bopp produced carbon monoxide (CO)
emissions equal to that given off by 5.5 billion cars every day.

Just like the results of an emission test help a mechanic diagnose an engine
problem, the results of NASA measurements help pin down where Hale-Bopp
and many other comets formed. When compared to the quantity of water in
the comet, the amount of CO indicates that comet Hale-Bopp was formed in
the region between Jupiter and
Neptune, according to recent observations.

"Comets are interesting because they are frozen relics from the formation of
our solar system, and by studying them, we can learn more about how we got
here," said Dr. Michael DiSanti of Catholic University and NASA's Goddard
Space Flight Center (Greenbelt, Md.). "Our observations of Hale-Bopp
indicate that comets now in the distant Oort cloud were originally part of
the solar system's ancient proto-planetary disk. It was thought that comets
could have formed in the cold, dense cloud of gas and dust that existed
before the proto-planetary disk formed. However, if this were so, we would
have seen even more carbon monoxide emission from Hale-Bopp. The amount
of carbon monoxide ice compared to water (12 percent) indicates that these
comets formed somewhere between the orbits of Jupiter and Neptune. We
hope to learn more about what was going on when the giant planets formed
by investigating the chemistry of this comet."

CO is a molecule made from a carbon and an oxygen atom. It is normally a
gas on Earth, and is monitored in vehicle emissions because it is toxic.

The new results were announced by a team of astronomers from Goddard,
Catholic University of America, Rowan University, Iona College, and Notre
Dame University. The researchers used an infrared spectrometer on a three-
meter telescope -- the NASA Infrared Telescope Facility (IRTF), Mauna Kea
Observatory, Hawaii -- to make the observations. "Our observational approach
combined with the unusually large size of Hale-Bopp permitted the first
definitive measurements of the amounts of carbon monoxide and water
present as ices in comets," said Dr. Michael Mumma of Goddard. The research
will be published in the June 17 issue of Nature.

Comets, nicknamed "dirty snowballs," are lumps of ice and dust a few miles
to a few tens of miles in diameter. Conditions were warmer in the
proto-planetary disk than in the gas and dust cloud that preceded it, due to
radiation from the nascent Sun. Since water ice freezes at higher
temperature than carbon monoxide ice, comets forming in the relatively
warm disk would have more water and less carbon monoxide, the researchers
believe. After the comets formed, gravitational pulls from gas giant planets
in the outer solar system cast them into cold storage about a trillion miles
from the Sun, in a roughly spherical region called the Oort cloud. Over the
ages, gravity from passing stars perturbs some of these comets, sending
them back toward the solar system as celestial visitors from our distant
past.

As a comet approaches the Sun, solar heat and radiation liberate gas and
dust from the comet's frigid surface, forming a cloud of material (the coma)
that is later pulled into the familiar comet tail by pressure from solar
wind and radiation. Gas molecules in the coma absorb light from the Sun and
emit it again as specific colors. The colors are unique for each chemical in
the gas, and serve as an optical "fingerprint" to identify various
substances,
including CO. Special instruments called spectrometers separate the emitted
light, much like a prism separates white light into a rainbow of distinct
colors, permitting the identification of these fingerprints.

As the comet nears the Sun, solar heat and light break down different
chemicals in its coma. This creates a distributed source of additional
carbon monoxide, making it difficult to determine the original amount
present as ice in the comet nucleus. The NASA spectrograph can detect the
amount of carbon monoxide near the comet nucleus as well as farther out in
its coma. "Because Hale-Bopp was so bright, we were able to observe it while
still very distant from the Sun before the distributed source was activated.
We were thus able to measure carbon monoxide emission solely from the
nucleus, and determine the true carbon monoxide to water ratio. Later,
as the comet approached the Sun, both sources were active and our
measurements then revealed the characteristic spatial signature of the
distributed source of CO," said Dr. Neil Dello Russo of Catholic University
and Goddard.

"There is another group of comets that may be more like the cold, dense
cloud that preceded the solar system disk.
Kuiper belt objects lie beyond
the orbit of Pluto, and probably formed from remnants at the fringes of the
proto-planetary disk. They are believed to be the source of comets with
short orbital periods and low inclinations to the ecliptic plane. This
region is remote from the Sun, and that part of the disk would have been
less influenced by radiation from the young Sun, so ices in Kuiper belt
objects are probably more like those in the original cloud from which the
solar system formed," said Mumma. "Future measurements may reveal this
difference."